This invention relates to emission control in automotive engines. Numerous proposals have been made for such control, including the use of "lean burn" mixtures of fuel/air. For example U.S. Pat. No. 4,368,707 (incorporated herein by reference) discloses a system wherein the ratio of fuel/air is varied by a servo valve in response to a control signal derived from engine power output.
However, there are problems in meeting emission control regulations under certain running conditions. Firstly, in the zero throttle condition, i.e., with manifold vacuum in excess of 20 in. Hg., the engine functions like a pump, and the lean burn mixture is ineffective. Combustion efficiency is poor and relatively large amounts of hydrocarbons may be released. Similarly, at low engine speeds, the lean burn mixture reduces the combustion temperature, once again adversely affecting combustion efficiency. Also, under deceleration conditions (reduced throttle) from speed, there is once again a departure from optimum burn. The system of U.S. Pat. No. 4,368,707 controls the fuel/air ratio to give optimum run quality. "Optimum run quality" means that for a given engine, the operating condition is maintained at a subjectively acceptable level, given tht excessively lean mixtures result in rough or uneven running characteristics. Where optimum exhaust emission control is achieved, the fuel/air mixture is close to the limit at which rough running results. According to U.S. Pat. No. 4,368,707 this is accomplished by feeding the final mixture control element (throttle valve) with two opposing signals, one causing enrichment on detection of a given deceleration rate and the second causing the mixture to go lean at a prechosen continuous rate. The result is that the rate of change of the fuel/air mixture is automatically proportional to the difference between the actual mixture and the desired mixture.